This invention relates to recombinant DNA technology. In particular, the invention concerns the cloning of nucleic acid encoding a multiple drug resistance protein of Aspergillus nidulans. 
Multiple drug resistance (MDR) mediated by the human mdr-1 gene product was initially recognized during the course of developing regimens for cancer chemotherapy (Fojo et al., 1987, Journal of Clinical Oncology 5:1922-1927). A multiple drug resistant cancer cell line exhibits resistance to high levels of a large variety of cytotoxic compounds. Frequently these cytotoxic compounds will have no common structural features nor will they interact with a common target within the cell. Resistance to these cytotoxic agents is mediated by an outward directed, ATP-dependent pump encoded by the mdr-1 gene. By this mechanism, toxic levels of a particular cytotoxic compound are not allowed to accumulate within the cell.
MDR-like genes have been identified in a number of divergent organisms including numerous bacterial species, the fruit fly Drosophila melanogaster, Plasmodium falciparum, the yeast Saccharomyces cerevisiae, Caenorhabditis elegans, Leishmania donovanii, marine sponges, the plant Arabidopsis thaliana, as well as Homo sapiens. Extensive searches have revealed several classes of compounds that are able to reverse the MDR phenotype of multiple drug resistant human cancer cell lines rendering them susceptible to the effects of cytotoxic compounds. These compounds, referred to herein as xe2x80x9cMDR inhibitorsxe2x80x9d, include for example, calcium channel blockers, antiarrhythmics, antihypertensives, antibiotics, antihistamines, immuno-suppressants, steroid hormones, modified steroids, lipophilic cations, diterpenes, detergents, antidepressants, and antipsychotics (Gottesman and Pastan, 1993, Annual Review of Biochemistry 62:385-427). Clinical application of human MDR inhibitors to cancer chemotherapy has become an area of intensive focus for research.
On another front, the discovery and development of antifungal compounds for specific fungal species has also met with some degree of success. Candida species represent the majority of fungal infections, and screens for new antifungal compounds have been designed to discover anti-Candida compounds. During development of antifungal agents, activity has generally been optimized based on activity against Candida albicans. As a consequence, these anti-Candida compounds frequently do not possess clinically significant activity against other fungal species such as Aspergillus nidulans. However, it is interesting to note that at higher concentrations some anti-Candida compounds are able to kill other fungal species such as A. nidulans and A. fumigatus. This type of observation suggests that the antifungal target(s) of these anti-Candida compounds is present in A. nidulans as well. Such results indicate that A. nidulans may possess a natural mechanism of resistance that permits them to survive in clinically relevant concentrations of antifungal compounds. Until the present invention, such a general mechanism of resistance to antifungal compounds in A. nidulans has remained undescribed.
The invention provides, inter alia, isolated nucleic acid molecules that comprise nucleic acid encoding a multiple drug resistance protein from Aspergillus nidulans, herein referred to as atrD, vectors encoding atrD, and host cells transformed with these vectors.
In another embodiment, the invention provides a method for determining the fungal MDR inhibition activity of a compound which comprises:
a) placing a culture of fungal cells, transformed with a vector capable of expressing atrD, in the presence of:
(i) an antifungal agent to which said fungal cell is resistant, but to which said fungal cell is sensitive in its untransformed state;
(ii) a compound suspected of possessing fungal MDR inhibition activity; and
b) determining the fungal MDR inhibition activity of said compound by measuring the ability of the antifungal agent to inhibit the growth of said fungal cell.
In still another embodiment the present invention relates to strains of A. nidulans in which the atrD gene is disrupted or otherwise mutated such that the atrD protein is not produced in said strains.
In yet another embodiment, the present invention relates to a method for identifiying new antifungal compounds comprising the use of atrD gene disruption or gene replacement strains of A. nidulans. 
The present invention provides isolated nucleic acid molecules that comprise a nucleic acid sequence encoding atrD. The cDNA (complementary deoxyribonucleic acid) sequence encoding atrD is provided in the Sequence Listing as SEQ ID NO: 1. The amino acid sequence of the protein encoded by atrD is provided in the Sequence Listing as SEQ ID NO: 2.
Those skilled in the art will recognize that the degenerate nature of the genetic code enables one to construct many different nucleic acid sequences that encode the amino acid sequence of SEQ ID NO: 2. The cDNA sequence depicted by SEQ ID NO: 1 is only one of many possible atrD-encoding sequences. Consequently, the constructions described below and in the accompanying examples for the preferred nucleic acid molecules, vectors, and transformants of the invention are illustrative and are not intended to limit the scope of the invention.
All nucleotide and amino acid abbreviations used in this disclosure are those accepted by the United States Patent and Trademark Office as set forth in 37 C.F.R. xc2xa71.822(b)(1994).
The term xe2x80x9cvectorxe2x80x9d refers to any autonomously replicating or integrating agent, including but not limited to plasmids, cosmids, and viruses (including phage), comprising a nucleic acid molecule to which one or more additional nucleic acid molecules can be added. Included in the definition of xe2x80x9cvectorxe2x80x9d is the term xe2x80x9cexpression vectorxe2x80x9d. Vectors are used either to amplify and/or to express deoxyribonucleic acid (DNA), either genomic or cDNA, or RNA (ribonucleic acid) which encodes atrD, or to amplify DNA or RNA that hybridizes with DNA or RNA encoding atrD.
The term xe2x80x9cexpression vectorxe2x80x9d refers to vectors which comprise a transcriptional promoter (hereinafter xe2x80x9cpromoterxe2x80x9d) and other regulatory sequences positioned to drive expression of a DNA segment that encodes atrD. Expression vectors of the present invention are replicable DNA constructs in which a DNA sequence encoding atrD is operably linked to suitable control sequences capable of effecting the expression of atrD in a suitable host. Such control sequences include a promoter, an optional operator sequence to control transcription, a sequence encoding suitable MRNA ribosomal binding sites, and sequences which control termination of transcription and translation. DNA regions are operably linked when they are functionally related to each other. For example, a promoter is operably linked to a DNA coding sequence if it controls the transcription of the sequence, or a ribosome binding site is operably linked to a coding sequence if it is positioned so as to permit translation.
The term xe2x80x9cMDR inhibition activityxe2x80x9d refers to the ability of a compound to inhibit the MDR activity of a host cell, thereby increasing the antifungal activity of an antifungal compound against said host cell.
In the present invention, atrD may be synthesized by host cells transformed with vectors that provide for the expression of DNA encoding atrD. The DNA encoding atrD may be the natural sequence or a synthetic sequence or a combination of both (xe2x80x9csemi-synthetic sequencexe2x80x9d). The in vitro or in vivo transcription and translation of these sequences results in the production of atrD. Synthetic and semi-synthetic sequences encoding atrD may be constructed by techniques well known in the art. See Brown et al. (1979) Methods in Enzymology, Academic Press, N.Y., 68:109-151. atrD-encoding DNA, or portions thereof, may be generated using a conventional DNA synthesizing apparatus such as the Applied Biosystems Model 380A,380B, 394 or 3948 DNA synthesizers (commercially available from Applied Biosystems, Inc., 850 Lincoln Center Drive, Foster City, Calif. 94404).
Owing to the natural degeneracy of the genetic code, the skilled artisan will recognize that a sizable yet definite number of nucleic acid sequences may be constructed which encode atrD. All such nucleic acid sequences are provided by the present invention. These sequences can be prepared by a variety of methods and, therefore, the invention is not limited to any particular preparation means. The nucleic acid sequences of the invention can be produced by a number of procedures, including DNA synthesis, cDNA cloning, genomic cloning, polymerase chain reaction (PCR) technology, or a combination of these approaches. These and other techniques are described by Maniatis, et al., Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Press, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y. (1989), or Current Protocols in Molecular Biology (F. M. Ausubel et al., 1989 and supplements). The contents of both of these references are incorporated herein by reference.
In another aspect, this invention provides the cDNA encoding atrD, which may be obtained by synthesizing the desired portion of SEQ ID NO:1 or by following the procedure carried out by Applicants. This procedure involved construction of a cosmid genomic DNA library from Aspergillus nidulans strain OC-1, a mutant derived from A42355. This library was screened for genes related to MDRs using a homologous probe generated by PCR. Degenerate PCR primers directed towards amplification of DNA sequences encoding highly conserved regions found in the ATP-binding domain of several MDR genes were synthesized. PCR using these primers and Aspergillus nidulans genomic DNA as template produced an approximately 400 base pair DNA fragment. The DNA sequence of this fragment was highly homologous to the ATP-binding region of several MDRs as predicted. This fragment was used as a hybridization probe to identify cosmid clones containing the entire atrD gene. A subclone from one such cosmid containing the entire atrD gene was sequenced to ascertain the entire sequence of atrD.
To effect the translation of atrD-encoding mRNA, one inserts the natural, synthetic, or semi-synthetic atrD-encoding DNA sequence into any of a large number of appropriate expression vectors through the use of appropriate restriction endonucleases and DNA ligases. Synthetic and semi-synthetic atrD-encoding DNA sequences can be designed, and natural atrD-encoding nucleic acid can be modified, to possess restriction endonuclease cleavage sites to facilitate isolation from and integration into these vectors. Particular restriction endonucleases employed will be dictated by the restriction endonuclease cleavage pattern of the expression vector utilized. Restriction enzyme sites are chosen so as to properly orient the atrD-encoding DNA with the control sequences to achieve proper in-frame transcription and translation of the atrD molecule. The atrD-encoding DNA must be positioned so as to be in proper reading frame with the promoter and ribosome binding site of the expression vector, both of which are functional in the host cell in which atrD is to be expressed.
Expression of atrD in fungal cells, such as Saccharomyces cerevisiae is preferred. Suitable promoter sequences for use with yeast hosts include the promoters for 3-phosphoglycerate kinase (found on plasmid pAP12BD (ATCC 53231) and described in U.S. Pat. No. 4,935,350, Jun. 19, 1990) or other glycolytic enzymes such as enolase (found on plasmid pAC1 (ATCC 39532)), glyceraldehyde-3-phosphate dehydrogenase (derived from plasmid pHcGAPC1 (ATCC 57090, 57091)), hexokinase, pyruvate decarboxylase, phosphofructokinase, glucose-6-phosphate isomerase, 3-phosphoglycerate mutase, pyruvate kinase, triosephosphate isomerase, phosphoglucose isomerase, and glucokinase. Inducible yeast promoters have the additional advantage of transcription controlled by growth conditions. Such promoters include the promoter regions for alcohol dehydrogenase 2, isocytochrome C, acid phosphotase, degradative enzymes associated with nitrogen metabolism, metallothionein (contained on plasmid vector pCL28XhoLHBPV (ATCC 39475), U.S. Pat. No. 4,840,896), glyceraldehyde 3-phosphate dehydrogenase, and enzymes responsible for maltose and galactose utilization (GAL1 found on plasmid pRY121 (ATCC 37658) and on plasmid pPST5, described below). Suitable vectors and promoters for use in yeast expression are further described by R. Hitzeman et al., in European Patent Publication No. 73,657A. Yeast enhancers such as the UAS Gal enhancer from Saccharomyces cerevisiae (found in conjunction with the CYCI promoter on plasmid YEpsecxe2x80x94hI1beta, ATCC 67024), also are advantageously used with yeast promoters.
A variety of expression vectors useful in the present invention are well known in the art. For expression in Saccharomyces, the plasmid YRp7, for example, (ATCC-40053, Stinchcomb et al., 1979, Nature 282:39; Kingsman et al., 1979, Gene 7:141; Tschemper et al., 1980, Gene 10:157) is commonly used. This plasmid contains the trp gene which provides a selection marker for a mutant strain of yeast lacking the ability to grow in tryptophan, for example ATCC 44076 or PEP4-1 (Jones, 1977, Genetics 85:12).
Expression vectors useful in the expression of atrD can be constructed by a number of methods. For example, the cDNA sequence encoding atrD can be synthesized using DNA synthesis techniques such as those described above. Such synthetic DNA can be synthesized to contain cohesive ends that allow facile cloning into an appropriately digested expression vector. For example, the cDNA encoding atrD can be synthesized to contain NotI cohesive ends. Such a synthetic DNA fragment can be ligated into a NotI-digested expression vector such as pYES-2 (Invitrogen Corp., San Diego Calif. 92121).
An expression vector can also be constructed in the following manner. Logarithmic phase Aspergillus nidulans cells are disrupted by grinding under liquid nitrogen according to the procedure of Minuth et al., 1982 (Current Genetics 5:227-231). Aspergillus nidulans mRNA is preferably isolated from the disrupted cells using the QuickPrep(copyright) mRNA Purification Kit (Pharmacia Biotech) according to the instructions of the manufacturer. cDNA is produced from the isolated mRNA using the TimeSaver(copyright) cDNA Synthesis Kit (Pharmacia Biotech) using oligo (dT) according to the procedure described by the manufacturer. In this process an EcoRI/NotI adapter (Stratagene, Inc.) is ligated to each end of the double stranded cDNA. The adapter modified cDNA is ligated into the vector Lambda ZapRII(copyright) using the Predigested Lambda ZapRII(copyright)/EcoRI/CIAP Cloning Kit (Stratagene, Inc.) according to the instructions of the manufacturer to create a cDNA library.
The library is screened for full-length cDNA encoding atrD using a 32P-radiolabeled fragment of the atrD gene. In this manner, a full-length cDNA clone is recovered from the Aspergillus nidulans cDNA library. A full-length cDNA clone recovered from the library is removed from the Lambda ZapRII(copyright) vector by digestion with the restriction endonuclease NotI which produces a DNA fragment encoding atrD. The atrD encoding fragment is subcloned into plasmid pYES2 for expression studies. In this plasmid the atrD gene is operably linked to the Saccharomyces cerevisiae GAL1 promoter at the 5xe2x80x2 end, and the yeast cyc1 transcription terminator at the 3xe2x80x2 end. This plasmid further comprises the ColE1 origin of replication (ColE1) which allows replication in Escherichia coli host cells, and the ampicillin resistance gene (Amp) for selection of E. coli cells transformed with the plasmid grown in the presence of ampicillin. The expression plasmid further comprises the yeast 2xcexc origin of replication (2xcexc ori) allowing replication in yeast host cells, the yeast URA3 gene for selection of S. cerevisiae cells transformed with the plasmid grown in a medium lacking uracil, and the origin of replication from the f1 filamentous phage.
In a preferred embodiment of the invention Saccharomyces cerevisiae INVSc1 or INVSc2 cells (Invitrogen Corp., Sorrento Valley Blvd., San Diego Calif. 92121) are employed as host cells, but numerous other cell lines are available for this use. The transformed host cells are plated on an appropriate medium under selective pressure (minimal medium lacking uracil). The cultures are then incubated for a time and temperature appropriate to the host cell line employed.
The techniques involved in the transformation of yeast cells such as Saccharomyces cerevisiae cells are well known in the art and may be found in such general references as Ausubel et al., Current Protocols in Molecular Biology (1989), John Wiley and Sons, New York, N.Y. and supplements. The precise conditions under which the transformed yeast cells are cultured is dependent upon the nature of the yeast host cell line and the vectors employed.
Nucleic acid, either RNA or DNA, which encodes atrD, or a portion thereof, is also useful in producing nucleic acid molecules useful in diagnostic assays for the detection of atrD MRNA, atrD cDNA, or atrD genomic DNA. Further, nucleic acid, either RNA or DNA, which does not encode atrD, but which nonetheless is capable of hybridizing with atrD-encoding DNA or RNA is also useful in such diagnostic assays. These nucleic acid molecules may be covalently labeled by known methods with a detectable moiety such as a fluorescent group, a radioactive atom or a chemiluminescent group. The labeled nucleic acid is then used in conventional hybridization assays, such as Southern or Northern hybridization assays, or polymerase chain reaction assays (PCR), to identify hybridizing DNA, cDNA, or RNA molecules. PCR assays may also be performed using unlabeled nucleic acid molecules. Such assays may be employed to identify atrD vectors and transformants and in in vitro diagnosis to detect atrD-like mRNA, CDNA, or genomic DNA from other organisms.
U.S. patent application Ser. No. 08/111,680 (now abandoned), the entire contents of which are hereby incorporated herein by reference, describes the use of combination therapy involving an antifungal agent possessing a proven spectrum of activity, with a fungal MDR inhibitor to treat fungal infections. This combination therapy approach enables an extension of the spectrum of antifungal activity for a given antifungal compound which previously had only demonstrated limited clinically relevant antifungal activity. Similarly, compounds with demonstrated antifungal activity can also be potentiated by a fungal MDR inhibitor such that the antifungal activity of these compounds is extended to previously resistant species. To identify compounds useful in such combination therapy the present invention provides an assay method for identifying compounds with Aspergillus nidulans MDR inhibition activity. Host cells that express atrD provide an excellent means for the identification of compounds useful as inhibitors of Aspergillus nidulans MDR activity. Generally, the assay utilizes a culture of a yeast cell transformed with a vector which provides expression of atrD. The expression of atrD by the host cell enables the host cell to grow in the presence of an antifungal compound to which the yeast cell is sensitive to in the untransformed state. Thus, the transformed yeast cell culture is grown in the presence of i) an antifungal agent to which the untransformed yeast cell is sensitive, but to which the transformed host cell is resistant, and ii) a compound that is suspected of being an MDR inhibitor. The effect of the suspected MDR inhibitor is measured by testing for the ability of the antifungal compound to inhibit the growth of the transformed yeast cell. Such inhibition will occur if the suspected Aspergillus nidulans MDR inhibitor blocks the ability of atrD to prevent the antifungal compound from acting on the yeast cell. An illustrative example of such an assay is provided in Example 3.
In order to illustrate more fully the operation of this invention, the following examples are provided, but are not to be construed as a limitation on the scope of the invention.